Background: Clinical studies have shown that palbociclib improves progression-free survival in hormone receptor–positive (HR+), human epidermal growth factor receptor 2–negative (HER2−) patients with advanced breast cancer (ABC). However, there are insufficient data on its use in a real-world setting in Japan. The aim of this study was to investigate the effectiveness, predictive factors, and safety of palbociclib among Japanese patients in routine clinical practice. Methods: Between December 1, 2017, and April 30, 2019, we recruited patients from 9 hospitals and retrospectively evaluated the data on HR+/HER2− patients with ABC who received palbociclib for at least 1 week. The correlation between time-to-treatment discontinuation (TTD) and clinical background was investigated via univariate and multivariate analyses using Cox hazards models. Results: A total of 177 women were available for analysis. Of these patients, 58 (33%) patients were treated with palbociclib with an aromatase inhibitor and 117 (66%) patients were treated with palbociclib and a selective estrogen receptor degrader. Approximately three-fourths of the patients (n = 130, 73%) received palbociclib as third- or later-line therapy. One-third of the patients had 3 or more metastatic sites (n = 59, 33%), and one-third of the patients had liver metastasis (n = 59, 33%). The median follow-up duration at the time of data cutoff was 8.9 months, the median TTD was 6.3 months, and the median overall survival was not reached. Liver metastasis (hazard ratio [HR]: 1.54 [95% confidence interval {CI}: 1.03-2.27]), high serum lactate dehydrogenase (LDH) level (>300 U/L) (HR: 2.58 [95% CI: 1.49-4.26]), and high neutrophil-to-lymphocyte ratio (NLR) (⩾3.0) (HR: 1.76 [95% CI: 1.13-2.69]) were significantly associated with shorter TTD. The most common hematologic adverse event was neutropenia, which occurred in 93% of the patients. Conclusion: Based on the results of the pivotal phase 3 trials, the median TTD recorded in this study was shorter than expected. Our results suggest that liver metastasis, serum LDH level, and NLR may be predictive factors for HR+/HER2− ABC treatment outcomes.

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